This invention relates to an apparatus for filtering high temperature gases, such as high pressure gases discharged from a circulating fluidized bed reactor. The filtration apparatus typically comprises porous ceramic filter elements. The present invention also related to a method for filtering high temperature gases containing solid contaminants and for efficiently removing accumulated solids from the filtering elements in a safe manner.
It is known in the prior art to use ceramic filters, in order to remove particulates from hot gas streams. It is e.g. known to use candle type ceramic filters (as shown in U.S. Pat. No. 4,869,207) support by a cooled tube sheet for cleaning hot gases. The size of such a filter unit is however presently limited, the practical limit for the diameter of a pressure vessel with candle type filter being about 3-5 m.
It is also known to use ceramic through flow filter tubes vertically supported by cooled support plates in filter units. The size of these filter unit housings are practically restricted as it is difficult to make large water cooled support plates having e.g. diameters larger than 2 m, due to expansion of the vessel itself and due to rigidity required of the support plate.
It is also known that it is essential to have the filters cleaned, e.g. after certain pre-determined operating periods, in order to be able to maintain the desired pressure drop. Commonly used methods of cleaning the filter employ a reverse directional pulse of gas for flushing the filter.
In order to provide larger filter units it has been suggested to provide a filtration housing having cooled walls and a plurality of tubes mounted horizontally within the dirty gas space in the filtration housing. The tubes reaching e.g. from one wall to the opposite wall within the filtration housing. If however large amounts of solid particulates are to be filtrated from the gas then particulate matter accumulating on the tubes may easily cause plugging problems. The filter units have to be pulsed frequently. Pulsing of filter tubes has to be done such that no damage is caused to the fragile ceramic tubes, which may be difficult to avoid. The arrangement of horizontal tube filters provides an advantage over conventional candle type filters in so far that larger filter units may be built, but still larger units with higher filtration area per filter housing volume and more easily to be cleaned filters are desired.
It has also been suggested to utilize porous monolithic ceramic filters providing a high filtration area per filter housing volume. The monolithic ceramic filters have a plurality of passageways extending longitudinally from inlet end to outlet end, but the passageways being plugged to prevent direct passage of the feed stock through the passageways from the inlet face (dirty gas side) to the outlet face (clean gas side) forcing clean gas to pass through the porous ceramic material into an adjacent passageway being connected to the clean gas side of the filter. The cleaning capacity of clusters of such monolithic ceramic filters is much higher than of conventional candle type or tubular ceramic filters. The monolithic ceramic filters thereby being less space consuming than conventional tubular or candle type ceramic filters.
The mounting of these monolithic elements in filtration vessels in high temperature surroundings and possible temperature variations has however led to very complicated and expensive constructions. When cleaning high temperature gases from combustors, gasifiers or the like, the whole filtration vessel construction is heated, and has to be made of expensive heat resistant material. It is therefore economically impractical to build large filtration units.
Side walls in a high temperature filtration unit, including monolithic filter elements, tend to "live", expand, when the unit is heated, whereby joints between monolithic filter elements and side walls also "live" and cause problems.
It is an object of the present invention to provide an improved apparatus for filtering high temperature gases.
It is especially an object of the present invention to provide a high temperature high pressure ceramic filtration unit for pressurized combustion, gasification or related processes to overcome aforementioned problems and to furnish an inexpensive, simple and reliable gas cleaning apparatus.
According to one embodiment of the present invention, an apparatus for filtering high temperature gases both from pressurized (i.e. superatmospheric pressure) systems and atmospheric systems, is provided. The filtration apparatus comprises following elements:
a generally upright outer vessel having a top, a bottom and a side wall; PA1 at least one generally upright inner vessel, being disposed within said outer vessel, said inner vessel having generally impervious peripheral walls preventing gas from flowing through said walls and dividing the gas space in said outer vessel into a dirty gas space and a clean gas space, PA1 a plurality of monolithic ceramic filter elements disposed in openings arranged in said peripheral walls of said inner vessel, allowing clean gas to flow through said filter elements from one side of the peripheral wall of said inner vessel to the other, and wherein PA1 said peripheral wall includes cooling elements, such as cooling tubes with a coolant such as water, steam or air. PA1 expansion of the inner vessel easily controlled; PA1 the support structure of the ceramic elements can be more easily arranged; PA1 the temperature of the filtration vessel materials may be held lower and more easily handled; PA1 the filtration system can be started up in a shorter time and can withstand higher temperature variations, PA1 temperature variations during back pulsing do not cause problems, and PA1 gas can be cooled simultaneously as it is cleaned.
In a preferred apparatus embodiment an upright inner vessel is constructed of water tube panels, in which adjacent vertical water tubes are connected to each other by fins. The inner vessel is disposed concentrically in the outer vessel and the monolithic ceramic filter elements are connected by their inlet dirty ends to openings in the water tube panels. The main part of the filter elements, with the clean gas ends, protrude through the water tube panels into the clean gas space, only a very small portion &lt;1/3 of the length of the filter element or almost nothing at all of the filter element protruding into the dirty gas space. Means for back pulsing the filter elements are provided in front of the clean gas ends of the filter elements.
In one exemplary embodiment of the filtration apparatus an inlet for dirty gas is provided in the top of the outer vessel, a support plate is provided in the upper part of the outer vessel, the support plate dividing the gas space in the outer vessel into a dirty gas side above the support plate and a clean gas side below said support plate. The upper end of the inner vessel is connected to an opening in the support plate, such that the main part of the inner vessel is disposed below the support plate. The interior of the inner vessel is connected through the opening in the support plate to the dirty gas side above the support plate. A clean gas space is prevailing on the outside of the inner vessel. The filter elements protrude from the dirty gas side of the inner vessel outward into the clean gas space outside the inner vessel. A clean gas outlet is provided in a side wall of the outer vessel below the support plate. A solid particle outlet is provided in the bottom of the inner vessel, the solid particle outlet being connected to a solid particle outlet in the bottom of the outer vessel.
Monolithic ceramic filter elements may be connected in different modes to the openings in the water tube panels or walls. The openings may be provided in broad fins between adjacent water tubes, the width of said broad fins being mainly of the same magnitude as the diameter of said filter elements, such that a filter element may be inserted in the opening. The filter elements being e.g. circular, square or polygonal in cross section. The monolithic ceramic filter elements may alternatively be connected to openings provided in a water tube panel by bending apart two adjacent or more water tubes to provide a distance between said water tubes corresponding to the diameter of said ceramic filter elements.
A layer of refractory lining may be provided to cover each side of the water tube panel, such that the ends of the ceramic elements and the surface of the layer of refractory lining form a mainly smooth outer and inner surface in the inner vessel.
According to another preferred embodiment of the present invention the apparatus may include an inner vessel wall in which openings for filter elements are provided in several parallel vertical rows at a distance from each other. The openings are provided one on top of the other at a distance from each other. Connecting elements are provided at least in the vertical parts of the openings in order to connect the filter elements to the peripheral wall construction, i.e. for providing a seat for the monolithic ceramic filter elements in the openings.
The vertical water tube panels, formed of e.g. 2-4 vertical water tubes connected by fins, and used to cool the peripheral wall have mainly the same horizontal width as is the length of a ceramic element and the panels are mounted between the vertical rows of filter elements, perpendicular to the wall plane. According to this embodiment the water tube panels are mainly at a 90.degree. angle to the plane of the inner surface of the inner vessel. The panels thereby do not form a conventional water tube walls in the vessel. E.g. in a cylindrical vessel the separate water tube panels form radially disposed cooling panels between the rows of filter elements.
In order to form a peripheral wall for the inner vessel additional elements, such as refractory lined metal plates or other refractory lined constructions may be used to combine adjacent water tube panels with each other and thus form a gas tight enclosure for the inner vessel. The water tube panels efficiently cool the connecting elements needed for locking the filter elements into the openings in the peripheral wall. The cooling may be still improved by connecting bars or fins between the cooling tubes and the connecting elements. The cooling of the connecting elements prevents undue expansion thereof and provides a reliable seat for the filter elements. Further the tube panels, which are disposed perpendicularly in the peripheral wall of the inner vessel, form stabilizing and supporting elements in the overall construction of the inner vessel.
In an alternative exemplary embodiment of the filtration apparatus there is an inlet for dirty gas in the side wall of the outer vessel, a support plate in the upper part of the outer vessel, the support plate dividing the gas space in the outer vessel into a clean gas side above the support plate and a dirty gas side below the support plate. The upper end of the inner vessel is connected to an opening in the support plate, such that the main part of the inner vessel is disposed below the support plate. The interior of the inner vessel is connected through the opening in the support plate to the clean gas side above the support plate and a dirty gas space is prevailing on the outside of the inner vessel. Thereby the filter elements protrude from the dirty gas side of the inner vessel inward into the clean gas space inside the inner vessel. A clean gas outlet is provided in the top of the outer vessel above the support plate, and a solid particle outlet is provided in the bottom of the outer vessel.
In still another alternative exemplary embodiment of the filtration apparatus there is an inlet for dirty gas in the top of the outer vessel, an inner vessel below the inlet for dirty gas and a guiding element between the inlet and the inner vessel, for guiding dirty gas flowing through the inlet into the outer vessel radially outward. A dirty gas space is provided on the outside of the inner vessel and a clean gas space on the inner side of the inner vessel. A solid outlet conduit is provided in the bottom of the outer vessel and a clean gas outlet conduit in the inner vessel.
According to the present invention it is easy to control the temperature of constructions around ceramic filter elements mounted in openings in a peripheral wall of a filtration chamber. The temperature of the peripheral wall of the inner filtration vessel, carrying the filter elements, is easy to predict at all times. Sudden high temperature peaks are avoided, thereby minimizing the danger of thermal shock related damages.
Other advantages arising from cooling the peripheral walls of the inner filtration vessel include:
Further the ceramic elements may be supported such that one end of the filter element or both ends of the filter element are free to thermal expansion, as the element is rather short compared to tubular ceramic filters needing supports at both ends. The monolithic ceramic filter elements may be packed in a very compact configuration permitting a smaller outer pressure vessel.
Also other advantages are achieved especially when back pulsing the ceramic filters, when the filter elements are disposed with their clean ends protruding rather deep, e.g. &gt;1/2 of them, into the clean gas space side e.g. inside the vessel. The high pressure cleaning pulse thereby compresses the portions of the ceramic filter elements being inside the tubular chamber from all sides, preventing mechanical breaking of the filter elements.